Abstract
Lateral movement from the principal trajectory, or “swing”, can be generated on a cricket ball when its seam, which sits proud of the surface, is angled to the flow. The boundary layer on the two hemispheres divided by the seam is governed by the Reynolds number and the surface roughness; the swing is fundamentally caused by the pressure differences associated with asymmetric flow separation. Skillful bowlers impart a small backspin to create gyroscopic inertia and stabilize the seam position in flight. Under certain flow conditions, the resultant pressure asymmetry can reverse across the hemispheres and “reverse swing” will occur. In this paper, particle image velocimetry measurements of a scaled cricket ball are presented to interrogate the flow field and the physical mechanism for reverse swing. The results show that a laminar separation bubble forms on the non-seam side (hemisphere), causing the separation angle for the boundary layer to be increased relative to that on the seam side. For the first time, it is shown that the separation bubble is present even under large rates of backspin, suggesting that this flow feature is present under match conditions. The Magnus effect on a rotating ball is also demonstrated, with the position of flow separation on the upper (retreating) side delayed due to the reduced relative speed between the surface and the freestream.
Highlights
The unique design of a cricket ball allows a bowler to influence both its lateral and vertical movement through the air
Cricket balls are constructed from a cork center and four quadrants of leather tightly stitched around it, forming a primary seam—which sits proud of the surface by 0.5 to 0.8 mm—and two internally stitched quarter seams, resulting in a surface that is almost flush [1]
This paper presents an investigation into the flow field surrounding a cricket ball using particle image velocimetry (PIV) in a low speed wind tunnel
Summary
The unique design of a cricket ball allows a bowler to influence both its lateral and vertical movement through the air. This angle is typically around β = 15◦ This creates asymmetry in the separation points of the viscous boundary layers on each hemisphere either side of the seam, resulting in a lateral aerodynamic force. The flow remains laminar on the smooth and polished NSS, and subsequently separates from the surface at θL ~80◦ This angle, and those below are quantified from experimental measurements on balls used in first-class match conditions by Scobie et al [5] at. The experiments used a scaled cricket ball at Re and k/d values typical of match conditions, exploring the role of the boundary layer and flow separation during reverse swing. The results are discussed in the practical context of a cricket match
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